Gravitational Waves From Bubble Universe Collisions?

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In the beginning was the bubble: Inflating, in a fraction of a second, from a grain smaller than an atom to a mini-verse the size of a softball. The seeds of all the elaborate particulars of today’s universe, from the vast cosmic web that links galaxy clusters all the way down to the motes of cosmic dust drifting past Earth, were wound up in tiny quantum fluctuations in that original bubble, just waiting for time, and gravity, to uncoil them.

That’s the story of inflation, and it’s the prevailing narrative for how our universe came to be. It has a lot going for it: It explains why the universe has such a uniform temperature, why spacetime appears to be flat, and why physicists can’t find any magnetic monopoles. In the last decade, inflation’s predictions have lined up neatly with observations from telescopes like

Planck
and
WMAP
, which have mapped miniscule deviations in cosmic microwave background radiation, an electromagnetic “echo” from near the time of the Big Bang. Though inflation has its critics, it remains the leading theory of how our universe came to be.

Follow inflation to what many theorists think is its logical conclusion, though, and things get very strange. That’s because many versions of inflation lead straight to a multiverse: that is, a cosmos in which our universe is just one of many universes, each with different laws and fundamental constants of physics. The idea is controversial, not least because there is no guarantee that we would ever be able to prove or disprove the existence of these other universes. Now, a team of theorists has shown that
a collision between universes would create gravitational waves that could imprint a unique polarization signal on the sky
, potentially providing observational evidence for the existence of other universes.

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Jonathan Braden, who worked on the paper while he was a graduate student at the University of Toronto, compares the multiverse to a pot of simmering water. As the water boils, air bubbles big and small spontaneously pop into existence and jiggle about. Now imagine that our entire known universe is one of those air bubbles, swimming through the “water” of the universe’s native vacuum energy, as other bubbles emerge around it. The analogy isn’t perfect: For one thing, the energy that drives the creation of new bubble universes isn’t thermal energy, like the heat of a stove, but the inevitable fluctuations that are built in to the principles of quantum mechanics. Even stranger, the “pot”—the space in which the bubble universes are emerging—is constantly getting bigger, and the water supply always being replenished.

In this ever-simmering universe, bubbles may occasionally bump in to each other. If our universe was part of such a collision some time in the distant past, it could leave a telltale circular “bruise” on the cosmic microwave background (CMB) radiation. Astronomers
first scanned
the CMB’s tiny temperature variation for this telltale mark back in 2011, using measurements from NASA’s Wilkinson Microwave Anisotropy Probe, but found nothing. A
second analysis
also came up empty-handed.

Now, Braden and his collaborators Dick Bond (University of Toronto) and Laura Mersini-Houghton (University of North Carolina-Chapel Hill) argue that, when it comes to finding evidence for bubble universe collisions, those temperature variations may be only half the picture. In a new paper, they show that the collision should also imprint a distinctive polarization pattern in the CMB. Like the signal famously
found
and
lost
by the BICEP2 experiment last year, which astronomers initially thought came from the inflation process itself, the polarization would take a form called
B-modes
, and would be generated by gravitational waves. But, unlike the primordial B-modes from inflation, which should be coming from everywhere at once, the collision signal would be localized to a single disk of sky.

The new prediction comes from considering nuances in the shape of colliding bubbles. Traditionally, researchers have approximated the bubbles as perfect spheres. In reality, though, those spheres would have little bumps and ridges. Braden compares each bubble to a raised relief globe: from far away, it looks perfectly smooth, but up close mountain ranges and valleys become visible.

“Those mountains and valleys begin to grow very quickly once the bubbles collide,” says Braden. In the disk where the two bubbles intersect, “It looks like someone just splatter-painted it–it’s like a Jackson Pollock painting.” Most exciting for scientists, the collision should also produce gravitational radiation. That radiation could be observed today as localized B-mode polarization that matches up with the temperature “bruise” in the CMB.

The odds of picking up such a signal from a telescope like BICEP2, which only observes a small piece of the sky, are low, points out Braden. That’s because the collision, if it happened, would have to be coincident with the telescope’s field of view. “Ideally an
all-sky satellite experiment
designed to very precisely measure polarization is the best hope,” says Braden. Today, Braden is working at University College London in the laboratory of Hiranya Peiris, who has participated in previous searches for evidence of bubble collisions. He anticipates that she and her colleagues will be eager to take up the search for the new polarization signal.

If the signal is a no-show, that doesn’t rule out the existence of other universes. But if observers do detect and confirm it, it would be revolutionary. “You might not see anything,” says Braden. “But if you do, it’s giving you an observational handle on physics you probably can’t get in any other way.”

Go Deeper
Editor’s picks for further reading

arXiv:
Eternal Inflation and Its Implications
Alan Guth, who pioneered the theory of cosmic inflation, provides an in-depth look at its implications for the creation of a multiverse of “bubble” or “pocket” universes.